It is known that particles in a sample can be characterized by illuminating the sample and measuring the light scattered by the particles. The particles of the sample are typically dispersed within a sample cell in a dispersant medium during measuring. The dispersant medium is typically air or water, and typically flows through the sample cell during measurement.
The correlation between light scattering and particle characteristics can be described by the well-known Mie solution to Maxwell's equations. Smaller particles tend to result in larger scattering angles, and larger particles result in smaller scattering angles. The light scattered at each of a range of angles from the sample can be used to determine, for example, a size distribution of the particles in the sample. Such a measurement may be referred to as a static light scattering (SLS) measurement.
It is important that the illuminating light beam and scattered light detectors are properly positioned and aligned with each other (along with any other optical elements in the system, such as lenses, mirrors, etc). The light source and detectors may be fixed to a common optical frame, which is housed within (or is part of) the instrument main body.
It is also important for such measurements that the walls of the sample cell are kept clean and are free from scattering centres. If the walls of the sample cell become dirty, they may scatter light, contaminating the signal at the detectors, and resulting in noise, and/or unreliable/inaccurate particle characterization.
Furthermore, it is desirable for the cell walls to be accurately positioned with respect to the illuminating beam and the other components of the instrument (for example any lenses and detector elements). The optical paths are typically designed around a known sample cell position, and may not be able to accommodate a sample cell position that is ill-constrained.
These requirements have previously been addressed by mounting the sample cell on a sample cell sub-assembly which can be removed from the main instrument body. The sample cell windows can subsequently be removed from the sub-assembly for cleaning. Once the sample cell assemblies are cleaned, they can be re-assembled in the sub-assembly, and the sub-assembly returned to the main instrument body.
A clear path is needed within the instrument main body for removing the sample cell sub-assembly. Furthermore, accurate positioning of the sample cell walls with respect to the instrument main body relies on two locating engagements. Firstly, the sub-assembly must accurately locate relative to the instrument main body. Second, the sample cell windows must accurately locate relative to the sample cell sub-assembly. These locating engagements may each be subject to some error, which is undesirable.
An instrument that solves or ameliorates at least some of the above mentioned problems is desired.